1. Field of the Invention
The present invention relates generally to a power supply, and more particularly, it relates to a power supply capable of protecting peripheral circuits when the electric device circuit is applied with over-voltage or the output terminal of the circuit is short-circuited, due to an abnormal operation in the output terminal of the circuit.
2. Description of the Related Art
In general, electric devices such as image displays are driven by the switching mode power supply (SMPS) that is high in efficiency and small and light in size.
FIG. 1 is a schematic block diagram showing an embodiment of the prior art switching mode power supply. With reference to FIG. 1, the switching mode power supply comprises a rectifying unit 11, an auxiliary power supply coordinating unit 13, an auxiliary power supply transformer 15, a controlling unit 17, a photo-coupler 19, a switching controlling unit 21, and a main power supply transformer 23.
The rectifying unit 11 rectifies AC power supplied from an external source into DC power and outputs the rectified power as a DC voltage. Generally, the rectifying unit 11 rectifies AC power supplied from an external source into DC power and generates DC 24V and DC 5V. Here, the DC 24V is boosted in the main power supply transformer 23, outputted to the electric device, and used as the operating power for the electric device. And, the DC 5V is used as the operating power for the controlling unit 17 and the switching controlling unit 21.
The auxiliary power supply coordinating unit 13 is inputted with the DC 5V rectified by the rectifying unit 11. Also, the auxiliary power supply coordinating unit 13 generates a pulse width modulation (PWM) signal and switching-controls the auxiliary power supply transformer 15 based on the generated PWM signal.
The auxiliary power supply transformer 15 is connected to the controlling unit 17 and a light-receiving transistor 19b of the photo-coupler 19. The auxiliary power supply transformer 15, switching-controlled by the auxiliary power supply coordinating unit 13, boosts the DC 5V inputted from the auxiliary power supply coordinating unit 13 to a suitable voltage and supplies the boosted voltage to the controlling unit 17 and the photo-coupler 19.
The auxiliary power supply transformer 15 provides the controlling unit 17 with the boosted voltage through a diode D1. The controlling unit 17 receives the boosted voltage from the auxiliary power supply transformer 15 and generates ON/OFF control signal. The photo-coupler 19 is driven by the generated ON/OFF control signal. That is, the ON/OFF control signal generated by the controlling unit 17 is applied to a light-emitting diode 19a of the photo-coupler 19, and hence the light-emitting diode 19a of the photo-coupler 19 emits the light according to the ON control signal generated by the controlling unit 17. The light-receiving transistor 19b is turned on according to the light emitted by the light-emitting diode 19a of the photo-coupler 19. If the light-receiving transistor 19b is turned on, the supply voltage Vcc boosted by the auxiliary power supply transformer 15 is supplied to the switching controlling unit 21.
If the switching controlling unit 21 is provided with the supply voltage Vcc boosted by the auxiliary power supply transformer 15, the switching controlling unit 21 switching-controls the main power supply transformer 23 so that the DC 24V rectified by the rectifying unit 11 can be boosted. The DC 24V rectified by the rectifying unit 11 is boosted by the main power supply transformer 23, and the boosted voltage is supplied through diode D2 to the electric device circuit.
However, according to the prior art power supply, if the controlling unit 17 generates ON control signal, the supply voltage Vcc from the auxiliary power supply transformer 15 is continuously applied to the switching controlling unit 21, whereby the switching controlling unit 21 continuously supplies the power to the electric device according to the applied voltage Vcc. In this case, if the voltage applied to the electric device circuit exceeds a given voltage due to surge voltage generated in the transforming process by the main power supply transformer 23, there is a possibility that the electric device circuit is damaged or cracked.